Voltmeter ammeter from indicators from tape recorders lighthouse. Pointer instruments - indicators. Why one device cannot measure a wide range of quantities

Measuring current is a fairly important procedure for calculating and testing electrical circuits. If you are creating a device with power consumption at the level of charging a mobile phone, the usual one is enough to measure.

A typical inexpensive household tester has a current measurement limit of 10 A.

Most of these devices have an additional connector for measuring larger quantities. When rearranging the measuring cable, you probably haven’t thought about why you need to organize an additional circuit, and why you can’t just use the mode switch?

Important! Without knowing it, you have activated the ammeter shunt.

Why can’t one instrument measure a wide range of quantities?

The operating principle of any ammeter (pointer or coil) is based on converting the measured value into its visual display. Pointer systems operate on a mechanical principle.

A current of a certain magnitude flows through the winding, causing it to deflect in the field of a permanent magnet. There is an arrow attached to the reel. The rest is a matter of technique. Scale, markings, etc.

The dependence of the deflection angle on the current strength on the coil is not always linear; this is often compensated by a specially shaped spring.

To ensure measurement accuracy, the scale is made with as many intermediate divisions as possible. In this case, to ensure a wide measurement range, the scale must be of enormous size.

Or you need to have several instruments in your arsenal: an ammeter for tens and hundreds of amperes, a regular ammeter, a milliammeter.

In digital multimeters the picture is similar. The more accurate the scale, the lower the measurement limit. And vice versa - an overestimated value of the limit gives a large error.

A scale that is too busy is inconvenient to use. A large number of positions complicate the design of the device and increase the likelihood of loss of contact.

By applying Ohm's law to a section of the circuit, you can change the sensitivity of the device by installing a shunt for the ammeter.

On the pages of Parkflyer, modelers often raise the topic of promptly checking the serviceability of the RU transmitter and its antenna, which is the most important point in the reliability of the interaction between the transmitter and the receiver during flights of RU models.
To check the serviceability of the transmitter and its antenna, I use a simple homemade Electromagnetic Field Indicator, which I made from a dial indicator of the recording level from an old tape recorder. The indicator turned out to be very small, smaller than a matchbox and easily fits in the breast pocket of a shirt, which allows you to monitor the radiation of the transmitter and the serviceability of its antenna at any time right in the field.

The dial indicator for recording a tape recorder is a microammeter with a deflection current of 50....100 µA.
To make the Indicator, in addition to the head, you need two microwave diodes; I used KD514A diodes. A half-wave section of a suitable wire Ø 1 mm is used as an antenna. For 2.4 GHz RU transmitters, the length of the segment is 60 mm. The circuit diagram of the device is simple.

Solder the diodes to the indicator terminals. This is what KD514A diodes look like.

Ready device.

The antenna is glued with epoxy not directly to the indicator body, although it is made of plastic, but through a piece of strip. The fact is that the instrument scale is drawn on a metal plate, which is attached to the back cover inside the case, and if the antenna is glued directly to the cover, it will be located in close proximity to the metal scale at a distance of 1.5 mm from it, separated by a plastic bottom. As a result, a small capacitance appears between the metal scale and the antenna (but the frequency is 2400 MHz!), which significantly reduces the sensitivity of the indicator - the arrow deviates by a smaller angle, and if you make a gap of 6...8 mm, then the capacitance becomes negligible and the arrow deviates by big angle. Therefore, I had to make a gap from a piece of slats. This nuance was revealed during the manufacture of the Field Indicator.

Here is a video showing the practical application of the Indicator.

To make a Field Indicator, any microammeter with a current of 50....100 µA is suitable, not necessarily from a tape recorder. This will only affect the size of the device.

Here are good M4206 100 µA heads, but they are currently difficult to find.

You can also use other microwave diodes, for example: KD503, D403, D405, D605, D20.

A good microwave diode is obtained from a GT346 transistor with a collector closed to the base.
It is located in the ancient SKD-24, is quite sensitive and operates up to 2.4 GHz and higher.
Happy flights and soft landing everyone!

♦ In the previous article: to control the charging current it is used ammeter for 5 - 8 amperes. An ammeter is a rather scarce thing and you can’t always find one for such a current. Let's try to make an ammeter with our own hands.
To do this, you will need a pointer measuring device of the magnetic-electric system for any current of the full deviation of the needle on the scale.

It is necessary to ensure that it does not have an internal shunt or additional resistance for the voltmeter.
♦ The measuring pointer device has an internal resistance of the movable frame and the current of the full deflection of the pointer. The pointer device can be used as a voltmeter (additional resistance is connected in series with the device) and as an ammeter (additional resistance is connected in parallel with the device).

♦ The circuit for the ammeter is on the right in the figure.

Additional resistance - shunt calculated using special formulas... We will make it in a practical way, using only a calibration ammeter on current up to 5 - 8 amperes, or by using a tester, if it has such a measurement limit.

♦ Let's assemble a simple circuit from a charging rectifier, a standard ammeter, a wire for a shunt and a chargeable battery. See the picture...

♦ A thick wire made of steel or copper can be used as a shunt. It’s best and easiest to take the same wire that was used to wind the secondary winding, or a little thicker.

You need to take a piece of copper or steel wire about 80 centimeters, remove the insulation from it. Make rings for bolt fastening at the two ends of the segment. Connect this segment in series with a reference ammeter.

Solder one end from our pointer device to the end of the shunt, and run the other along the shunt wire. Turn on the power, set the charge current using the regulator or toggle switches according to the control ammeter - 5 amp.
Starting from the soldering point, run the other end from the pointer device along the wire. Set the readings of both ammeters to the same level. Depending on the frame resistance of your pointer gauge, different pointer gauges will have different lengths of shunt wire, sometimes up to one meter.
This, of course, is not always convenient, but if you have free space in the case, you can carefully place it.

♦ The shunt wire can be wound into a spiral as in the figure, or in some other way depending on the circumstances. Stretch the turns a little so that they do not touch each other, or put rings made of vinyl chloride tubes along the entire length of the shunt.

♦ You can first determine the length of the shunt wire, and then use insulated wire instead of bare wire and wind it in bulk onto the workpiece.
You must select carefully, performing all operations several times, the more accurate the readings of your ammeter will be.
The connecting wires from the device must be soldered directly to the shunt, otherwise the device arrow will read incorrectly.

♦ The connecting wires can be of any length, and therefore the shunt can be located anywhere in the rectifier body.
♦ It is necessary to select a scale for the ammeter. The ammeter scale for measuring direct current is uniform.

Many home electricians are dissatisfied with industrial production testers, so they think about how to, as well as how to improve the functionality of the industrial production tester. For this purpose, a special shunt can be made.

Before you begin, you should calculate the shunt for the microammeter and find a material with good conductivity.

Of course, for greater measurement accuracy, you can simply purchase a milliammeter, but such devices are quite expensive, and they are rarely used in practice.

Recently, testers designed for high voltage and resistance have appeared on sale. They do not require a shunt, but their cost is very high. For those who use a classic tester made in Soviet times, or use a homemade one, a shunt is simply necessary.

Selecting a current ammeter is not an easy task. Most devices are produced in the West, in China or in the CIS countries, and each country has its own individual requirements for them. Also, each country has its own permissible values ​​of direct and alternating current, requirements for sockets. In this regard, when connecting a Western-made ammeter to domestic equipment, it may turn out that the device cannot correctly measure current, voltage and resistance.

On the one hand, such devices are very convenient. They are compact, equipped with a charger and easy to use. A classic dial ammeter does not take up much space and has a visually clear interface, but it is often not designed for the existing voltage resistance. As experienced electricians say, there are “not enough amperes” on the scale. Devices designed in this way necessarily require shunting. For example, there are situations when you need to measure a value up to 10a, but there is no number 10 on the instrument scale.

Here are the main ones disadvantages of a classic factory ammeter without a shunt:

• Large error in measurements;
• The range of measured values ​​does not correspond to modern electrical appliances;
• Large calibration does not allow small quantities to be measured;
• When trying to measure a large resistance value, the device goes off scale.

A shunt is necessary in order to correctly measure in cases where the ammeter is not designed to measure such quantities. If a home craftsman often deals with such quantities, it makes sense to make a shunt for an ammeter with your own hands. Shunting significantly improves the accuracy and efficiency of its work. This is an important and necessary device for those who often use the tester. It is usually used by owners of the classic 91s16 ammeter. Here are the main advantages of a homemade shunt:

Manufacturing procedure

Even a freshman at a vocational school or a novice amateur electrician can easily handle making a shunt on his own. If connected properly, this device will greatly increase the accuracy of the ammeter and will last a long time. First of all, it is necessary to calculate the shunt for a DC ammeter. You can learn how to make calculations via the Internet or from specialized literature addressed to home electricians. You can calculate the shunt using a calculator.

To do this, you just need to substitute specific values ​​into the finished formula. In order to use the calculation scheme, you need to know the real voltage and resistance for which a particular tester is designed, and also imagine the range to which you need to expand the capabilities of the tester (this depends on which devices a home electrician most often has to deal with ).

Perfect for making such materials:

• Steel clip;
• Roll of copper wire;
• Manganin;
• Copper wire.

You can purchase materials in specialized stores or use what you have at home.

In fact, a shunt is a source of additional resistance, equipped with four clamps and connected to the device. If steel or copper wire is used to make it, do not twist it into a spiral.

It is better to carefully lay it in the form of “waves”. If the shunt is sized correctly, the tester will perform much better than before.

The metal used to make this device must conduct heat well. But inductance, if a home electrician is dealing with the flow of a large current, can negatively affect the result and contribute to its distortion. This also needs to be kept in mind when making a shunt at home.

If a home electrician decides to purchase a commercially available ammeter, he should choose one with a fine calibration because it will be more accurate. Then, perhaps, you won’t need a homemade shunt.

When working with the tester, you should follow basic safety precautions. This will help prevent serious injury caused by electrical shock.

If the tester systematically goes off scale, you should not use it.

It is possible that the device is either faulty or is not able to show the correct measurement result without additional equipment. It is best to purchase modern, domestically produced ammeters, because they are better suited for testing new generation electrical appliances. Before you start working with the tester, you should carefully read the operating instructions.

A shunt is a great way to optimize the work of a home electrician when testing electrical circuits. In order to make this device with your own hands, you will only need a working industrial production tester, available materials and basic knowledge in the field of electrical engineering.

Visibility is a big deal. So popular wisdom says: “It’s better to see once than to hear a hundred times.” And in electronics, where the ongoing processes in the operation of a particular device are often confirmed indirectly, or even generally implied and even taken on faith, it is generally difficult to overestimate the visual display. It is not for nothing that oscilloscopes are so revered among radio amateurs, giving them the opportunity to “look” even into the process. But I won’t talk about the complex - I’d like to deal with the simple ones. I have assembled almost a dozen different chargers, and to charge batteries I increasingly use a simple laboratory power supply that has an output voltage and current. The measuring heads clearly inform how many volts and milliamps go to the battery being charged. But it’s not possible to use them everywhere; even the smallest of them will often still be prohibitively large for many amateur radio homemade products. But dial indicators from tape recorders and other radio devices of the last century, which have not been sold out in the bazaars to this day, will be just right here. Here are some of them:

Designed for operation in DC circuits, at any scale position. Total deflection current (depending on model) 40 - 300 µA. Internal resistance 4000 Ohm. Scale length - 28 mm, weight 25 g.

Designed to work with the scale in a vertical position. Deviation current 220 - 270 µA. Internal resistance 2800 Ohm. Dimensions 49 x 45 x 32 mm. Scale length - 34 mm.

designed to work at any scale position. The total deviation current is no more than 250 µA. Internal resistance 1000 Ohm. Dimensions 21.5 x 60 x 60.5 mm. Weight 30 gr. These indicators and others like them are united by:

• small size
• simplicity of design
• low cost
• and, of course, the operating principle

The operating principle is based on the interaction of two magnetic fields. The fields of a permanent magnet and the field formed by a current passing through a frameless frame, which consists of a large number (115 - 150) turns of copper wire with a diameter of only 8 - 9 microns. Without delving into the nuances, we can name two main actions that need to be performed in order to make it possible to use the existing indicator:

1. Equip it with a shunt or additional resistance (used to change the upper limit of measurement), depending on how you will use it (voltmeter / ammeter).
2. Make a new scale.

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